In order to follow a specified track by a light beam spot when carrying out recording and reproducing of information for an optical disk, a tracking control for controlling a light beam spot position has been carried out. The tracking control detects a tracking error which is a radial difference between the track and the light beam spot on the optical disk, and is carried out based upon the detected tracking error. In order to detect the tracking error, a push-pull method is mostly used for an optical disk, especially on which recording will be carried out, from the standpoint of efficiency of using a light beam.
However, the position detection method by the push-pull method has a disadvantage. It is that a spot difference is detected by an error detector in spite of irradiating the light beam spot in the center of the track in the case of a slant of a disk, etc.
In this way, an offset of .DELTA.V occurs in the tracking error signal when the spot difference occurs as shown in FIG. 3(a). A point where the tracking error signal becomes 0 is regarded as the center of the track, and the tracking control is made so as to control the position of the light beam spot. Therefore, in this case, the light beam spot control is carried out regarding not O point which shows that the true spot difference is equal to 0 but O' point which is off-centered as the center of the track, thereby preventing information from being correctly recorded and reproduced.
In addition, the offsets change as reflectance of the disk and the output of the laser beam change. For example, if in FIG. 3(b), the laser output increases to a times the output shown in FIG. 3(a), positive and negative amplitude of the tracking error signal become a times. Since the O point which shows the center of the true track is the middle point of the positive and negative amplitude, as apparent from FIG. 3(b), the offset becomes a times. For this reason, in order to carry out the accurate tracking control, the offset should be compensated as required in accordance with the disk reflectance and the laser output.
The following description will discuss a conventional example of the offset compensation method for the tracking error signal.
As to the first conventional example, there exists a method which has been disclosed in Japanese Laid-Open Patent Application No. 258232/1989 (Tokukaihei 1-258232). FIG. 4 shows the arrangement. An optical disk 1 is rotated by a drive motor 2. An optical pick-up 3 moves radially on the optical disk 1. A tracking error which is a radial difference on the optical disk 1 between the track and the light beam spot is detected by an optical system (not shown) in the optical pick-up 3, and becomes a tracking error signal TE via a tracking error detection circuit 4. The tracking error signal TE is sent to an amplifying circuit 5 and supplied to an addition circuit 6. An HF signal (information reproducing or "readback" signal) which is simultaneously obtained from the optical system (not shown) in the optical pick-up 3 is supplied to a head amplifier 12 via an HF signal detection circuit 11, and to a signal processing circuit 14 after being amplified. The HF signal is branched and its DC component is picked out by an LPF (low-pass filter) 13 so as to be adjusted for a suitable level by a variable resistor VR for a level adjustment. The DC component of the HF signal is added to a tracking error signal TE' in the addition circuit 6.
Since the DC component of the HF signal fluctuates in proportion to variations in the laser output and the reflectance of the optical disk 1, the offsets of the tracking error signal TE' are compensated by means of the DC component of the HF signal in the first conventional example. In this way, after the tracking error signal TE' has been compensated in the addition circuit 6, a compensation for stabilization of a tracking control system is made in a phase compensation circuit 8 by means of an amplifier 7 for gain adjustment. A tracking coil 10 is driven by a tracking coil driving circuit 9 and the tracking control is carried out so that the light beam spot follows the track.
In the second conventional example, a method shown in FIG. 5 is adopted. Here, for convenience of explanation, those members of the second conventional example that have the same arrangement and function, and that are mentioned in the aforementioned first conventional example are indicated by the same reference numerals and description thereof is omitted.
In FIG. 5, the total amount of a reflected light from the optical disk 1 is detected by the optical system (not shown) in the optical pick-up 3, and becomes a total signal TS via a total signal detection circuit 15. The total signal TS is amplified in the head amplifier 16 and supplied to a divider 17. As shown in FIGS. 3(a) and 3(b), the amplitude and offset of the tracking error signal TE increase or decrease as the laser power or the reflectance of the optical disk 1 increases or decreases. At the same time, the total signal TS also increases or decreases accordingly. For this reason, when the tracking error signal TE' is standardized by dividing it by the total signal TS' in the divider 17, the amplitude and offsets of the tracking error signal TE' is always kept constant, and the standardized tracking error signal TE' is supplied to the addition circuit 6. Moreover, a constant offset compensation voltage E is applied to the addition circuit 6 via the variable resistor VR.
In this way, when the tracking error signal TE' is divided by the total signal TS', the amplitude of the tracking error signal TE' becomes constant regardless of a fluctuation in the laser output or the reflectance. As a result, since a constant amount of the offset occurs, a constant level of the offset compensation voltage is applied to the addition circuit 6.
However, the above conventional arrangement has the following problem.
Namely, since the offset compensation cannot be carried out at the time of recording by the art of the first conventional example (FIG. 4), the art cannot be applied to the optical disk 1 where information is recorded.
This problem is clarified by considering that the HF signal is outputted only at the time of reproducing, it is not clear whether or not the HF signal is outputted at the time of recording, and if outputted, it is not guaranteed that the DC component reflects the reflectance of the optical disk 1 and the laser output.
The art of the second conventional example (FIG. 5) is applicable to the optical disk apparatus where information is recorded, but has the problem that the divider 17 is necessary.
In general, since the divider 17 is expensive, it is an undesirable unit from a viewpoint of lowering the cost of the optical disk apparatus.